You can't nominalize a noun, as 'nominalize' means 'to make into a noun'.This の is a substitution for が

の can be substituted for the subject-marker が (but not the conjunction が) and does so in fixed antiquated expressions, and in modern usage in order to prevent multiple が from causing confusion or to tighten the relationship between the noun and the verb.

SomeCallMeChris wrote:の can be substituted for the subject-marker が (but not the conjunction が) and does so in fixed antiquated expressions, and in modern usage in order to prevent multiple が from causing confusion or to tighten the relationship between the noun and the verb.

It's not this fixed; の can substitute for が in any relative clause and there doesn't need to be any big reason for it.